Two short stanzas, and a lot packed into them. The first sets up a bitter joke: a gun "made an excellent sermon." A sermon is meant to teach or persuade, and here the gun does that job without saying a word, which fits the title, "Louder than words." What it teaches is "the difference between us." The poem doesn't say who the two sides are, but the gun makes the gap feel real, sharp and unavoidable.
The second stanza turns inward. The speaker admits, "I am afraid I have forgot / how not to see." The word "afraid" works two ways: it's a polite apology, and it's also real fear. Having been shown the difference, the speaker can't go back to not noticing it. The mood is cold, uneasy and quietly regretful. There's no shouting, just a calm voice describing a loss of innocence, where seeing something clearly feels more like a burden than a gift.